Sudden voltage drop or power failure of mobile information apparatuses such as cellular phone and laptop computer causes not only an interruption of the functions of such equipment, but also erasure of information stored in the memory, thus resulting in serious troubles. As countermeasures against such troubles, various capacitors such as electrolytic capacitor and electric double layer capacitor (EDLC) and secondary batteries have been used to provide backup power. Mobile communication apparatuses such as cellular phone and PDA are required to make intermittent operations that cause voltage variations, and use power sources such as electrolytic capacitor to compensate for the variations.
Among such power sources, electric double layer capacitor and other capacitors of high capacity are particularly advantageous since the capacity per unit volume is as high as several hundreds or several thousands times that of conventional electrolytic capacitors. To make use of the high capacity, these capacitors have been widely used in the form of coin type and others, for backup power sources of portable devices such as cellular phone. In order to satisfactorily deal with instantaneous variable load by making use of the high capacity, it is important to have a high power.
A structure of capacitor having high capacity and high output power is disclosed, for example, in Japanese Unexamined Patent Publication No. 8-78057, and a cylindrical capacitor of 18650 size having the structure disclosed in the publication has been commercialized. Capacitors using organic electrolyte having high capacity and high power of cylindrical structure based on organic electrolyte have also been commercialized by other manufacturers.
In the meantime, there have been amazing advancements to make mobile information apparatuses such as cellular phones and laptop computers smaller in size and higher in functions, which make it necessary to support the higher data processing speed brought about by the higher functions and reduce the power consumption. Lower operating voltage and higher operating speed increase the importance of transient response performance to intermittent load, thus leading to ever greater variations in load on the power source.
As a result, small capacitors having lower ESR (equivalent series resistance) and higher capacity are required. In the case of data transmission via cellular phone, for example, since load width is increasing to the order of milliseconds due to the higher functions, the capacitors such as electrolytic capacitors that are used at present are reaching their limits of capacity, and accordingly there are increasing demands for improvements in the transient response performance of the power source.
As a measure to circumvent the problem of shortage in capacity, an attempt has been made with respect to the thickness of the electrode active material layers, for example, as described in Japanese Unexamined Patent Publication No. 8-45793. Although decreasing the thickness of the electrode active material layers is advantageous because it gives high power to the cell, it also has a problem of decreasing discharging capacity. The publication of the invention described above proposes a cell having high discharge capacity and high power characteristic that are achieved by combining electrodes having electrode active material layers of the thickness 300 μm and 20 μm so as to solve the problem described above.
Capacitors having high capacity such as electric double layer capacitor have overwhelming advantage over the electrolytic capacitor and the like in terms of capacity per unit cell volume. However it is not sufficient for a backup power source that is required to accommodate variable load of short load width on the order of microseconds. This is because, in the case of such applications as the load width is limited to a particular short period of time, importance is placed on balancing the capacity and the impedance in accordance to the load width rather than on high capacity. This is because the capacity and impedance determine the balance between the voltage drop caused by ESR and the voltage drop caused by the discharge of the electrical charge. Capacitors used at present are designed in such a manner as capacity and impedance are too high for the range of load width on the order of milliseconds, resulting in unsatisfactory balance which is difficult to improve. As a consequence, it is necessary to decrease the capacity and impedance.
There is increasing demand for improvements in the transient response performance of the power source so as to accommodate such large variable load as the load width is on the order of milliseconds due to the higher functions of mobile devices. To meet this demand, it is necessary to improve the balance between the voltage drop due to ESR and the voltage drop due to the discharge of the electrical charge, by setting such balance between capacity and impedance, different from those conventional electrolytic capacitors and EDLC, that corresponds to the load width on the order of milliseconds.